Yosemite 10.10.5

I'm so grateful you included so much information in your post it makes it so much easier to give the answer you want.

If your Mac can upgrade further than El Capitan then the App Store will refuse you the download.

Check here,

How to upgrade to macOS High Sierra - Apple Support

and if your Mac is not in the list for High Sierra, then you can get El Capitan,

How to upgrade to OS X El Capitan - Apple Support

You can check for Mojave here.

How to upgrade to macOS Mojave - Apple Support

Check to make sure your applications are compatible.        

Application Compatibility

El Capitan 10.11 Compatibility information 

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.